The phosphoinositide 3-kinase (PI3K) pathway is among the most commonly activated in human cancer and the importance in carcinogenesis is well established. (Samuels Y and Ericson K. Oncogenic PI3K and its role in cancer. Current Opinion in Oncology, 2006; 18:77-82) Initiation of signaling begins with the phosphorylation of phosphatidylinositol-4,5-bisphosphate (PIP2) to produce phosphatidylinositol-3,4,5-P3 (PIP3). PIP3 is a critical second messenger which recruits proteins that contain pleckstrin homology domains to the cell membrane where they are activated. The most studied of these proteins is AKT which promotes cell survival, growth and proliferation.
The PI3K family consists of 15 proteins that share sequence homology, particularly within their kinase domains, but have distinct substrate specificities and modes of regulation. (Vivanco I and Sawyers CL. The phosphatidylinositol 3-kinase-AKT pathway in human cancer. Nature Reviews Cancer, 2002; 2:489-501) Class I PI3Ks are heterodimers consisting of a p110 catalytic subunit complexed to one of several regulatory subunits collectively referred to as p85 and have been the most extensively studied in the context of tumorgenesis. The class 1A PI3K catalytic subunits comprise the p110α, p110β, and p110δ isoforms, which associate with one of five different regulatory subunits encoded by three separate genes. A single class 1B PI3K catalytic isoform p1110γ interacts with one of two associated regulatory subunits. (Crabbe T, Welham M J, Ward S G, The PI3k inhibitor arsenal: choose your weapon Trends in Biochem Sci, 2007; 32:450-456) Class 1 PI3Ks are primarily responsible for phosphorylating the critical PIP2 signaling molecule.
The link between the PI3K pathway and cancer was confirmed by a study which identified somatic mutations in the PIK3CA gene encoding the p110α protein. Subsequently, mutations in PIK3CA have been identified in numerous cancers including colorectal, breast, glioblastomas ovarian and lung. In contrast to PIK3CA, no somatic mutations in the β isoform have been identified. However, in overexpression studies the PI3Kβ isoform has been implicated as necessary for transformation induced by the loss or inactivation of the PTEN tumor suppressor both in vitro and in vivo. (Torbett N E, Luna A, Knight Z A, et al., A chemical screen in diverse breast cancer cell lines reveals genetic enhancers and suppressors of sensitivity to PI3K isotype-selective inhibition. Biochem J 2008; 415:97-110; Zhao J J, Liu Z, Wang L, Shin E, Loda M F, Roberts T M, The oncogenic properties of mutant p110a and p110b phosphatidylinositol 3-kinases in human mammary epithelial cells. Proc Natl Acad Sci USA 2005; 102:18443-8) Consistent with this finding, overexpression of the PIK3CB gene has been identified in some bladder, colon, glioblastomas and leukemias and siRNA mediated knockdown of p110β in glioblastoma cell lines results in suppression of tumor growth in vitro and in vivo. (Pu P, Kang C, Zhang Z, et al., Downregulation of PIK3CB by siRNA suppresses malignant glioma cell growth in vitro and in vivo. Technolo Cancer Res Treat 2006; 5:271-280) More recent data using shRNA demonstrated that downregulation of p100β and not p110α resulted in PI3K pathway inactivation and subsequent inactivation of tumor cell growth in PTEN deficient cancers cells both in vitro and in vivo. (Wee S, Wiederschain, Maira S-M, Loo A, Miller C, et al., PTEN-deficient cancers depend on PIK3CB. Proc Natl Acad Sci 2008; 105:13057-13062) Consistent with a role of PIK3CB signaling in PTEN null tumors, p110β was reported to be essential to the transformed phenotype in a PTEN-null prostate cancer model. (Jia S, Liu Z, Zhang S, Liu P, Zhang L, et al., Essential roles of PI(3)K-p110b in cell growth, metabolism and tumorgenesis. Nature 2008;10:1038) Taken together, these findings indicate PI3K p110β as a promising target in cancer therapy.
Clinally, the loss of PTEN expression in the germ line is the cause of the majority of cases of Cowden's syndrome, a multiple hamartomas syndrome that includes throid neoplasias (benigh and malignant) as part of the phenotype. Liaw, D., Marsh, D. J., Li, J., et al., 1997. Germline mutations of the pten gene in cowden disease, an inherited breast and thyroid cancer syndrome. Nat. Genet. 16, 64-67. PTEN-null cancers rely almost exclusively on PI3K-beta for growth signaling, thus an inhibitor will alleviate symptoms of Cowden Syndrome. Motoyasu Saji and Matthew D. Ringel. The PI3K-Akt-mTOR pathway in initiation and progression of thyroid tumors; Molecular and Cellular Endocrinology 321 (2010) 20-28.